Photosynthesis: a set of chain reactions that convert light energy into chemical energy. Photosynthesis also produces energy-rich carbohydrates like starch. Photosynthesis occurs in the chloroplast of a plant cell... more

What's in the Story?

It's a cloudy morning and you are halfway through the race, your feet already hurting from running on the pavement and your energy is fading fast. Suddenly, the clouds clear and the sun breaks through and shines down on your skin. You feel a rush of new energy that powers you through to finish the race.

Sunlight can make you feel good, especially on a cold day, but it can't give you energy...right? Animals eat food to gather energy and plants use sunlight to make energy, but wouldn't it be better if organisms could use both energy sources? If you were running out of food you could just sit in the sun to gather energy, saving your food for a rainy day. In the PLOS ONE article “Towards a Synthetic Chloroplast,” scientists tried to create an organism that used both food and sunlight to make energy.

Assembly Required

The problem is tricky; how would you go about making an organism that gets energy from both food and sunlight? We may not have instructions to this puzzle, but there are clues.

When you are exhausted, you can eat and drink for energy. But plants don’t seem to eat anything. Instead, they receive energy through water and sunlight. This is because plants and animals obtain energy using different organelles.

Animal cells use mitochondria to convert energy into food, and plant cells use chloroplasts to convert light into energy through photosynthesis. Putting these two organelles in the same cell would be like having two restaurants on the same street corner--they might not get along. So how did scientists figure out a way to combine both organelles in one cell? By learning a little bit about how chloroplasts and mitochondria came to exist in the first place.

Where Did They Come From?

Have you ever been eating a bowl of ice cream when your mom tells you “you are what you eat”? Well, you are definitely not made of ice cream, but "you are what you eat" may have been true for some cells a long time ago.

The idea is that there were large cells roaming around back then, eating smaller cells for food. Some of these small cells could not be properly broken down and digested by the large cells. This made it so the small cells could settle down to live within these large cells. This idea is called endosymbiosis, where endo- means within or inside, and -symbiosis means living together. So endosymbiosis means that two cells are living together, with one inside the other.

In this situation, both cells benefitted from the presence of the other. The large cells acted as houses for the small cells, and the small cells provided energy to the large cells. These small cells are believed to be the ancestors to what we now know as chloroplasts and mitochondria. We believe this because chloroplasts and mitochondria hold their own DNA that is unique from that of the large cells.

Endosymbiosis is thought to be a rare event in nature though, because most cells have defenses against invading cells. But using this idea helped the scientists come up with innovative ways they might be able to get a cell that has mitochondria to accept cyanobacteria, a special type of bacteria that has chloroplasts.

Invasion of the Green Aliens!

The body is like a well-protected castle, where foreign invaders are not allowed entry and are killed if they make it inside. So how could you get living bacteria into the body and help it survive? The scientists came up with three different methods of entry for the cyanobacteria.

First, the scientists directly injected the cyanobacteria into the middle of the animal cell because it allowed the bacteria to avoid the exterior defenses of the animal cell. For the other two methods, the scientists needed to upgrade the bacterial cells to survive the defense of the animal cells. They did this by strengthening the defenses of the cyanobacteria cells against the animal cell defenses.

This is like creating two perfect soldiers that can never outfight one another. The strengthened bacteria were then either allowed to invade the fish cells by themselves, or to be swallowed by specific fish cells that eat bacteria. The scientists hoped the cyanobacteria would survive and grow within the fish cells.

Plant-imals?

Though you may be picturing fish with plant-like skin, the scientists were not able to make the plant-animal hybrid. The scientists were hoping to see the cyanobacteria survive and reproduce within the animal cells. If the cyanobacteria were able to reproduce and increase in number, then maybe they could produce energy for the host cell and be considered similar to chloroplasts. But, this is not what the scientists found.

The cyanobacteria that had been swallowed by the special bacteria-eating cells had died after a few days. The introduced bacteria were not able to reproduce much within the cell either. Injecting the cyanobacteria was also unsuccessful in the long run, but the results of the experiment were promising.

The cyanobacteria injected in the zebrafish cells were able to grow and reproduce for twelve days, but eventually the molecules that color the fish’s skin blocked the cyanobacteria from gathering sunlight. Luckily, this short-term progress is enough to give the scientists hope. Maybe with a lot more research, we can one day reach the dream of powering up with chloroplasts in our skin.

Additional images from Wikimedia Commons.

Danio rerio (zebrafish) by Monte Westerfield via the National Institute of General Medical Science.

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Be part of Ask A Biologist

by volunteering, or simply sending us feedback on the site. Scientists, teachers, writers, illustrators, and translators are all important to the program. If you are interested in helping with the website we have a Volunteer page to get the process started.